In semiconductor package designs, semiconductor dies are mounted onto a substrate, and then encapsulated in a mold cavity. As semiconductor packages get thinner and thinner, molding cavity and molding gaps are so stretched that only a very thin layer of molding compound covers the die(s). Therefore, any variation of compound thickness will result in package warping. In addition, mismatch of coefficient of thermal expansion (CTE) between various materials inside a thin package can cause huge warpage.
Thin semiconductor packages employ thin dies and leadframes. As a result, when the die and leadframe thickness gets thinner, they are more prone to flexing during the assembly process. FIG. 1 is an exemplary thin semiconductor package 1 that includes a split pad 2, a lead terminal 3, a die 4, a bond wire 5, and an encapsulant 6. Since the gap between the pad 2 and the upper molding surface (not shown) is very narrow, the mold flow will cause the die 4 to flex (from 4 to 4′) and the die pad 2 to shift.
Currently, attempts have been made to remedy the warpage problems for semiconductor packages. For example, U.S. Pat. No. 6,331,448 discloses a semiconductor package that includes a leadframe with offsets from a major plane at a non-perpendicular angle thereto. The offsets are an attempt to prevent warpage, bowing, skewing, or other distortions of the semiconductor package 1 when subjected to the high temperature or changes in temperature. As another example, U.S. Pat. No. 6,020,221 discloses a semiconductor device that comprises a stiffener member, where the stiffener member is attachable to the semiconductor package substrate for purposes of minimizing package warpage. Currently, however, issues concerning flexing problems of the die 4 and/or the die-attach pad during the molding process are not adequately addressed.
There have also been attempts to remedy the displacement of assembly components during the molding process of a semiconductor package. For example, U.S. Pat. No. 6,028,368 discloses a semiconductor device with potting resin structures that are affixed to a plurality of predetermined faces of the leadframe so as to limit the shifting of the die and leadframe structures during the molding process. However, these potting structures may not able to stop the vertical movement of the die and leadframe completely.
Another example of problems in semiconductor packages concerns the encapsulation of dies mounted on a flexible tape substrate. When positioned in a mold tooling, as a molding compound flows into the mold chamber and around the mounted die, the flexible tape can flutter and become warped or bent resulting in a bowed die structure, and an uneven layer of encapsulant formed around the die structure, which can lead to structure failure. U.S. Patent Application Publication No. 2005/0023562 A1 discloses a semiconductor device that comprises a pair of semiconductor dies mounted on opposing side of a flexible tape substrate, the outer surfaces of the dies having one or more standoffs disposed thereon. The standoffs can be brought into contact with an inner surface of the mold plates of a mold tooling when the device is positioned between the mold plates to maintain the flexible tape substrate in a centralized position within a mold chamber and inhibit the tape from bending as a molding compound flows into the chamber during encapsulation. This may be perfectly suitable for flexible substrate when the semiconductor device is bulky. As for thin semiconductor packages, however, both the warpage and die/pad flexing have to be taken into consideration.